Proton exchange membrane (PEM) fuel cells are largely considered the main fuel cell technology candidate for, among others, portable and automotive transportation applications. A typical PEM fuel cell generally includes a membrane electrode assembly (MEA), a pair of gas diffusive backing layers positioned adjacent to each side of the MEA, and a pair of current-collecting flow plates positioned adjacent to each backing layer. The MEA, which is further composed of a solid polymer electrolyte membrane having an anode on one of its faces and a cathode on the opposite face, is the PEM fuel cell's main source of electrochemical activity where a usable electrical current that flows from the anode to the cathode is generated and intermittently utilized to power an external device. Such electrochemical activity is the result of catalyst-driven oxidation and reduction reactions that occur when a fuel is supplied to the anode side of the MEA and an oxidant is supplied to the cathode side. In many instances a large number of PEM fuel cells may be assembled in series to form what is known as a fuel cell stack to generate greater electrical current outputs. For example, stack arrangements of PEM fuel cells can generate a cumulative electrical current output capable of powering automotive devices such as electrical motors and certain auxiliary fuel cell stack equipment. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
The solid polymer electrolyte membrane of the MEA may be formed from a proton conductive and electrically insulative polymer material. The membrane can thus provide a conductive pathway for ion transport between the anode and the cathode, can direct the flow of an electrical current through an external device in order for that current to travel from the anode to the cathode and thus complete the electrochemical cell, and separate the reactant gases supplied to the anode and the cathode. A popular polymer material used to make the solid polymer electrolyte membrane of a PEM fuel cell is a perfluorosulfonic acid ionomer such as, for example, those manufactured by DuPont and sold under the trade name Nafion®. The other two main components of the MEA—namely, the anode and cathode—are each typically formed from an ionically and electrically conductive binder material mixed with finely divided catalyst particles that are often supported on larger carbon particles. These catalyst particles, which are oftentimes precious metal particles such as those of platinum and ruthenium, catalyze the oxidation and reduction half-reactions that occur at the anode and cathode, respectively.
The operation of many kinds of PEM fuel cells involves supplying hydrogen gas to the fuel cell anode and oxygen gas in the form of either air or pure oxygen to the fuel cell cathode. The anode functions to dissociate the incoming hydrogen gas molecules into protons and electrons. The presence of an electrochemical gradient within the MEA for each of these newly-formed charged particles causes them to move towards the cathode, albeit by different pathways. The protons, as alluded to before, migrate from the anode to the cathode through the solid polymer electrolyte membrane. The electrically insulative nature of the membrane, however, is not amenable to electron transport and thus forces the electrons to flow through an external circuit in order to reach the cathode. It is the purpose of the cathode to then facilitate the reaction of the arriving protons and electrons with supplied oxygen gas to form water. But to keep the MEA operating efficiently in such a manner over an extended period of time is somewhat dependent on the ability to maintain a certain level of hydration in the membrane for optimized proton conductivity while at the same time not flooding the anode and cathode. Indeed, many PEM fuel cell water management controls seek to achieve optimum hydration of the MEA by balancing the generation of water at the cathode, the removal of water from both the anode and cathode, and the relative humidity of the supplied reactant gas streams.
One particular water management issue that can affect the performance of the MEA is hydrogen starvation. Hydrogen starvation generally occurs when the flow of hydrogen gas to the anode is blocked by water accumulation at the anode or at the anode-side flow plate. This accumulation of excess water can be caused by the external humidification of the hydrogen reactant stream and/or by the back diffusion of water from the cathode to the anode. The occurrence of hydrogen starvation is problematic because it can initiate the formation of localized cathode cells near the anode. These localized cells form, in the absence of hydrogen, as a result of oxygen cross-over from the anode to the cathode and the lack of lateral proton transport due to the anisotropic electrical properties of the solid polymer electrolyte membrane. The most available source of protons for these oxygen-rich localized cathode cells now happens to be through the oxidation of carbon materials and/or oxidation of water on catalyst (oxygen evolution reaction) in the cathode. The phenomenon of hydrogen starvation thus promotes performance degradation of the cathode as a result of carbon corrosion.
Possible solutions for mitigating the problem of hydrogen starvation include (i) frequent flushing of the anode compartment to remove any accumulated nitrogen and/or water, (ii) use of an oxygen evolution catalyst that decreases the partial current for carbon oxidation in the cathode, and (iii) use of non-carbon catalyst supports in the cathode. But unfortunately all of these options have significant operational or practical drawbacks. It is therefore desirable to devise an alternative mechanism for addressing the hydrogen starvation problem.